Such a film is known, for instance, from EP-A-105 629, in which the materials used as thermoplastic polymer include polypropylene, polyethylene, polyethylene terephthalate, polycaprolactam and polyvinylidene fluoride. Depending on their pore dimensions, microporous films are more or less permeable to gases or liquids. They find application in many fields, for instance in filtration of solids, ultra-filtration of colloidal mixtures, as diffusion barrier or separation membrane in electrolytic cells, as selective cleaning filter for liquids, in which capacity they may either be filled or unfilled, and in clothing.
The known films have the disadvantage that their applicability is restricted by the properties of the polymers. This restriction may for instance, originate in the absence of certain desirable properties, such as dimensional stability at higher temperatures, good thermal and/or electrical conductivity, good mechanical properties, such as a high stiffness and a high strength, and resistance to chemical influences. At temperatures above their softening point, and in particular above their melting point, films from thermoplastic polymers undergo distinct changes in a number of essential product properties, for instance, shape and dimensions, porosity and mechanical properties such as strength and modulus. Thus, polyethylene starts to lose its dimensional stability at temperatures from about 80.degree. C., its softening point, while its melting point as a rule is between about 120.degree. and 150.degree. C., so that a film from this polymer can be used only at lower temperatures.